1. Field of the Invention
The present invention relates to an image forming apparatus having a plurality of image forming stations each including an image forming means disposed around an image bearing member.
2. Related Background Art
FIG. 2 shows a laser beam printer which has four image forming stations and which can output an image in response to color decomposing signals of an original from an image reader (not shown) and to which the present invention is applicable. FIG. 2 illustrates an example of laser beam printers having a plurality of light scanning means. In this laser beam printer, there are provided four image forming stations each of which has an image forming means (conventional electrophotographic process) disposed around an image bearing member comprising an electrophotographic photosensitive member, and images formed on the image bearing members at the respective image forming stations are sequentially transferred onto a transfer sheet carried by a belt-shaped moving body moving adjacent to the image bearing members.
More specifically, photosensitive drums 1M, 1C, 1Y, 1K are disposed in image forming stations Pm, Pc, Py, Pk for magenta, cyan, yellow, black colors, respectively, and are rotated in clockwise directions, respectively. Around the photosensitive drums 1M, 1C, 1Y, 1K, there are disposed image forming means comprising corona chargers 2M, 2C, 2Y, 2K, optical scanning devices (as laser beam scanning means) 3M, 3C, 3Y, 3K, developing devices 4M, 4C, 4Y, 4K, and cleaning devices 5M, 5C, 5Y, 5K, respectively.
Further, a transfer portion 6 forming a part of the image forming means has an endless belt (transfer sheet conveying means) 6a which is common to all of the image forming stations, and transfer corona dischargers 10M, 10C, 10Y, 10K, and a full-color image is obtained by sequentially transferring color images formed on the photosensitive drums onto a transfer sheet P supported on the transfer belt 6a. The transfer belt 6a is urged against the photosensitive drums by respective urging members made of elastic sheets 10MA, 10CA, 10YA, 10KA.
The transfer sheet P is supplied from a sheet cassette 7, and, after the transfer process, the sheet P is forcibly separated from the transfer belt and is fed to an ejection tray 9 through a fixing device 8.
Each of the optical scanning devices 3M, 3C, 3Y, 3K is constituted by a laser device acting as a light source, a rotary polygonal mirror for scanning the laser beam, an f.theta. lens for focusing the scanning beam onto the generatrix on a surface of the corresponding photosensitive drum, a reflection mirror for deflecting a bundle of light beams, and a beam detecting device for detecting a specific position of the scanning beam (all of these elements are not shown).
An operation timing in the image formation means of such image forming apparatus is shown in FIG. 3 as a timing chart.
In FIG. 3, a level "1" denotes an operative condition, and a level "0" denotes an inoperative condition. In order to eliminate the unnecessary movements in the image forming apparatus, it is considered that, when a copy start key is depressed, the photosensitive drums can be rotated sequentially in order from the first image forming station, and, when the image formation and the transferring of each image to the transfer sheet are finished, the photosensitive drums in respective stations can be sequentially stopped. On the other hand, when the copy start key is depressed, the transfer belt 6a carrying the transfer sheet thereon continues to move until all of the transfer processes are completed.
In such movements, there arises a condition that one of the photosensitive drums and the transfer belt is stopped and the other continues to move. If such condition occurs, since the transfer belt 6a is abutted against the photosensitive drums 1M, 1C, 1Y, 1K by means of the urging members 10MA, 10CA, 10YA, 10KA, respectively, when only one of these two elements is stopped, there arise a relative movement between these elements, with the result that the photosensitive drums and/or transfer belt are damaged, thus worsening the image quality.